Precision keynotes design of laser facility

The ultimate purpose of the National Ignition Facility (NIF) is to focus 192 laser beams, with an accuracy a fraction of the width of a human hair, on a target the size of an aspirin.

This daunting challenge suggests the degree of precision that was necessary to design and construct the $2.4 billion facility at the U.S. Department of Energy's Lawrence Livermore National Laboratory in Livermore, Calif. The NIF will perform fusion energy experiments by compressing and heating fusion fuel capsules in order to produce thermonuclear ignitions with a peak power of 500 trillion W.

The NIF will support electro-optic materials development. Because it will mimic and measure important aspects of a nuclear explosion, it also will enable the U.S. to retain its nuclear device stockpile without the need to perform underground nuclear tests.

Under construction since 1997, the facility is expected to conduct its first experiments by next month.

The complex consists of three connected structures. The primary buildings are the 275,000-sq.-ft. Laser and Target Building and the 33,000-sq.-ft. Optics Assembly Building. The Laser Building has two main bays, each 450 ft. long, 104 ft. wide, and 50 ft. high, that require precise temperature control. A deviation of more than half a degree from 68 F will result in improper focus of the laser beams. "We don't want temperature excursions to cause thermal creep," says NIF site manager Vaughn Draggoo.

The Target building, which has 6-ft.-thick concrete walls to prevent the escape of high-energy neutrons, houses a 33-ft.-diameter aluminum cylinder within which the target is located

More than 750 rose-colored, transparent mirrors focus the l92 laser beams on the target. The mirrors made it possible to use a laser building only half as long as would have been necessary otherwise, according to NIF spokesperson David Schwoegeler.

The project's key designers and contractors include Los Angeles-based AC Martin Partners as prime A/E and San Diego-based Nielsen Dillingham Builders as contractor for the Optical Assembly Building. Parsons Engineers, Pasadena, Calif., was the A/E and Greeley, Colo.-based Hensel Phelps Construction Co. the contractor, for the laser building. Jacobs Engineering Group of Pasadena was the construction manager for both buildings. Jacobs is also general contractor for the beampath and associated utility systems within the laser building.

Optical focus

"The design challenge was to recognize that NIF is basically an optical instrument," says Draggoo. "Because of that, the structure needs to be rigid and to meet certain earthquake standards."

Wayne Saxerud, operations manager for Hensel Phelps, said a critical aspect of the construction was to reduce the amount of vibration that could be transmitted into the laser building. Supports for the laser beam pathway rest on the building's 4-ft.-thick, heavily reinforced floor slab.

Kelly Turner, project manager for Nielsen Dillingham, says a major challenge was coordination with other contractors to keep the project moving.

NIF's control system integrates more than 40,000 control points to manage the 192 laser beams, which are delivered in a pulse that lasts only 25 nanoseconds, or 25 one-thousandths of a billionth of a second. The results of the fusion must be recorded within a few billionths of a second.

Construction at NIF is scheduled to continue until the end of this decade. Remaining work will involve the installation of instrumentation and the fitup of existing structures.